1. Field of the Invention
This invention relates to metal coated filaments and to a process and an apparatus for their continuous production.
2. Description of the Prior Art
Filaments comprising non-metals and semi-metals, such as carbon, boron, silicon carbide, polyester, nylon, aramid, cotton, rayon, and the like, in the form of monofilaments, yarns, tows, mats, cloths and chopped strands are known to be useful in reinforcing metals and organic polymeric materials. Articles comprising metals or plastics reinforced with such fibers find wide-spread use in replacing heavier components made up of lower strength conventional materials such as aluminum, steel, titanium, vinyl polymers, nylons, polyester, etc., in aircraft, automobiles, office equipment, sporting goods, and in many other fields.
A common problem in the use of such filaments, and also glass, asbestos and others, is a seeming lack of ability to translate the properties of the high strength filaments to the material to which ultimate and intimate contact is to be made. In essence, even though a high strength filament is employed, the filaments are merely mechanically entrapped, and the resulting composite pulls apapt or breaks at disappointingly low applied forces.
The problems have been overcome in part by depositing a layer or layers of metals on the individual filaments prior to incorporating them into the bonding material, e.g., metal or plastic. Metal deposition has been accomplished by vacuum deposition, e.g., the nickel on fibers as described in U.S. Pat. No. 4,132,828; and by electroless deposition from chemical baths, e.g., nickel on graphite filaments as described in U.S Pat. No. 3,894,677; and by electrodeposition, e.g., the nickel electroplating on carbon fibers as described in Sara, U.S. Pat. No. 3,622,283 and in Sara, U.S. Pat. No. 3,807,996. When the metal coated filaments of such procedures are twisted or sharply bent, a very substantial quantity of the metal flakes off or falls off as a powder. When such metal coated filaments are used to reinforce either metals or polymers, the ability to resist compressive stress and tensile stress is much less than what would be expected from the rule of mixtures, and this is strongly suggestive that failure to efficiently reinforce is due to poor bonding between the filament and the metal coating.
It has now been discovered that if electroplating is selected and if an amount of voltage is selected and used in excess of that which is required to merely dissociate (reduce) the electrodepositable metal ion on the filament surface, a superior bond between filament and metal layer is produced. The strength is such that when the metal coated filament is sharply bent, the coating may fracture, but it will not peel away. Moreover, continuous lengths of such metal coated filaments can be knotted and twisted without substantial loss of the metal to flakes or powder. High voltage is believed important to provide or facilitate uniform nucleation of the electrodepositable metal on the filament, and to overcome any screening or inhibiting effect of materials absorbed on the filament surface.
Although a quantity of electricity is required to electrodeposit metal on the filament surface, an increase in voltage to increase the amperes may cause the filaments to burn, which would interrupt a continuous process. The aforesaid Sara Pat. No. 3,807,966, uses a continuous process to nickel plate graphite yarn, but employs a plating current of only 2.5 amperes, and long residence times, e.g. 14 minutes, and therefore low, and conventional voltages. In another continuous process, described in U.K. Pat. No. 1,272,777, the individual fibers in a bundle of fibers are electroplated without burning them up by passing the bundle through a jet of electrolyte carrying the plating material, the bundle being maintained at a negative potential relative to the electrolyte, in the case of silver on graphite, the potential between the anode and the fibers being a conventional 3 volts.
The present invention provides an efficient apparatus to facilitate increasing the potential between anode and the continuous filament cathode, since it is a key aspect of the present process to increase the voltage to obtain superior metal coated fibers. In addition, since it permits extra electrical energy to be introduced into the system without burning up the filaments, residence time is shortened, and production rates are vastly increased over those provided by the prior art. As will be clear from the detailed description which follows, novel means are used to provide high voltage plating, strategic cooling, efficient electrolyte-filament contact and high speed filament transport in various combinations, all of which result in enhancing the production rate and quality of metal coated filaments. Such filaments find substantial utility, for example, when incorporated into thermoplastic and thermoset molding compounds for aircraft lightning protection, EMI/RFI shielding and other applications requiring electrical/thermal conductivity. They are also useful in high surface electrodes for electrolytic cells. Composites in which such filaments are aligned in a substantially parallel manner dispersed in a matrix of metal, e.g., nickel coated graphite in a lead or zinc matrix are characterized by light weight and superior resistance to compressive and tensile stress. The apparatus of this invention can also be employed to enhance the production rate and product quality when electroplating normally non-conductive continuous filaments, e.g., polyaramids or cotton, etc., if first an adherent electrically conductive inner layer is deposited, e.g., by chemical means, on the non-conductive filament.